[tex-k] ligatures and explicit kerns

[Karl Berry]

Hi Stephan,

    ligatures won't be inserted in words that do contain explicit kerns.
    ...
    shelf\/fulfi

It would be unexpected, at least by me, for a kern in one position
within a word to disable ligatures everywhere within the word, not just
at the point of the kern.  I agree that the text you quoted can be read
that way; repeating it here:

  Appendix H points out that ligatures are put into a hyphenated
  word that contains no ``explicit kerns,'' and an italic correction
  is an explicit kern.

but I would suggest interpreting it as a more limited statement:
ligatures are avoided at the point of the explicit kern.  (After all, TB
is not intended to be a legal document.)  At most there might be a
clearer statement in TB, if you want to make a report for DEK's
consideration; obviously the actual behavior is not going to be changed.

    I didn't find the place in appendix H the quote refers to, so
    pointers would be appreciated.

I do not see a single "place" that is being referred to; as far as I can
tell, it's rather a consequence of all the multifarious rules in App. H.
(But maybe I'm missing it, I did not reread every word [again].)
Perhaps the most direct statement is this one, talking about what
"hyphenating a word" actually means (namely, inserting discretionaries):

  \ddanger Trial words ...
  If other hyphenation points are found, one or more discretionary items
  are inserted in the word; ligatures and implicit kerns are reconstituted
  at the same time.

The first statement is a negative construction, the second a positive
one.  Always tricky to compare.

Best,
Karl